Machinery arrangements for marine vessels conventionally comprise one or more combustion units, such as diesel engines, oil-fired boilers, incinerators, etc. The combustion units usually have independent exhaust gas duct assemblies, whereby also the cleaning of exhaust gases takes place separately for each combustion unit.
Gases coming from oil-fired boilers are sometimes referred to as flue gases. Gases from diesel engines are mostly called exhaust gases. In the following the general term exhaust gas will be used with regard to gases from all relevant types of combustion units.
Reduction of exhaust gas emissions can be obtained by improving the performance of the combustion units, by using cleaner fuel, or by cleaning the exhaust gases.
In previously known solutions, with regard to cleaning exhaust gases, machinery arrangements have been provided with exhaust gas cleaning systems for nitric oxides (NOx).
Presently, however, due to increasing demands to reduce exhaust gas emissions, especially in port conditions and near the coast, there will be a need to clean sulphuric oxides (SOx) emanating from sulphur rich fuels, such as conventional heavy fuel oil.
Normally exhaust gas is led into an exhaust gas cleaning system, i.e. a scrubber unit in which a scrubbing medium, such as an alkaline washing solution, is sprayed into the exhaust gas, whereby the alkaline components react with acidic components of the exhaust gas. The scrubbing medium is usually supplied from a so-called process tank. The scrubbing medium absorbs SOx, other components, as well as heat from the exhaust gas flow.
Scrubber units are normally very large and have a considerable height. Furthermore, they are usually installed as more or less a last component at the end of an exhaust gas duct system. In marine vessels exhaust gas is usually led vertically out from the stack, whereby a scrubber unit has to be placed very high up in the structures of the marine vessel.
Sea water scrubbers have been used for sulphur removal in stationary applications and are now considered also for marine vessels.
Sea water scrubbers utilise the natural alkalinity of sea water to keep the pH at a sufficiently high level for SOx removal. Since alkalinity is limited, big relative flow volume flows are needed in order to achieve an efficient scrubbing process. This results in high energy consumption with due to large amounts of water that have to be pumped upwards to a considerable height, particularly when sea water is used as a scrubbing medium on a marine vessel.